Until recently, poly(alkylene) carbonates have had limited commercial application. They have been used as sacrifice polymers in the electronics industry but in few other applications. Other applications of these polymers have been limited by their relative thermal instability and mechanical properties.
The present inventors have realised that these poly(alkylene carbonates) offer environmentally friendly potential. The use of carbon dioxide in the formation of poly(alkylene carbonates) provides a useful sink for carbon dioxide and therefore these polymers offer an environmentally friendly alternative to fossil fuel based materials such as a polyolefin. There are therefore significant benefits to using PACs industrially.
As noted above, commercial applications of some poly(alkylene carbonates) are limited by their low glass transition temperature(Tg) and low thermal stability. Furthermore, thermal decomposition of these polymers occurs at rather low temperatures, e.g. at 150-180° C. for poly(propylene carbonate) (PPC). These two properties severely limit the processability of PACs on a commercial scale. Methods for improving the properties of PACs so as to enhance the applicability of PACs are therefore sought. End capping of the poly(alkylene carbonate) is one route which has been investigated and, to date, some progress has been achieved.
In U.S. Pat. No. 4,066,630, end capping of poly(alkylene carbonate) is suggested through the addition of agents such as isocyanates, maleic anhydride and carboxylic acid mono-anhydrides. The addition takes place at ambient temperature in a solvent and therefore involves reaction of the hydroxyl end groups of the poly(alkylene carbonate) with a reactive group in the end capping agent. The result is an increase in the decomposition temperature of the polymer.
Thus, the use of maleic anhydride as an end capping group is known in the art and has been found to improve thermal behaviour in these materials. The main idea behind end capping is that the presence of the end capping group prevents the “unzipping” reaction which can causes chain scission and hence thermal degradation of the polymer.
There remains, however, a need for new ways of improving the thermal stability of these environmentally friendly polymers. Also, the present inventors sought modification of PACs to improve other properties thereof, in particular their mechanical properties and optical properties, e.g. in relation to UV light. PACs have potential application in the replacement of polyolefins. It is well known that polymers are susceptible to degradation from UV light and hence additives are conventionally added to stop this happening.
The present inventors have surprisingly established that effective modification of PACs can be achieved by the addition of pyromellitic dianhydride or related polycarboxylic compounds. Surprisingly, these modifying compounds can provide much improved properties over the use of maleic anhydride, e.g. in terms of thermal properties, mechanical properties and optical properties.
In U.S. Pat. No. 4,686,274, certain poly(alkylene carbonate) polyols are modified through contact with a wide variety of modifying agents. The idea is to effect a transesterification reaction. This patent refers to low molecular weight PACs having Mw values of less than 5000 g/mol.
US2012/0059078 describes a poly(alkylene carbonate) for a sheet which can include a filler, flexibiliser and lubricant.
The present invention relies on the modification of the PAC via a compounding step in which the PAC is compounded together with a modifying agent, preferably by melt compounding at elevated temperature.
It is stressed therefore that the inventors consider modification of the PAC to occur via various means. Whilst end capping of the polymer chain is one such mechanism, it is believed that the use of a multicarboxylic acid or disguised carboxylic acid modifying group allows other mechanisms to come into play. Without wishing to be bound by any theory, it is suggested that such mechanisms could be for example coupling together polymer chains, optionally degraded polymer chains, to linear or branched structures. The compounded PAC of the invention, prepared using the modifying agents of the invention, surprisingly offers a commercially more attractive polymer than traditional end capping using mono- or dicarboxylic compounds such as maleic anhydride.